Controller pattern for route handlers in Express - Mini Project: Build & Apply

Controller pattern for route handlers

📖 Scenario: You are building a simple Express web server that manages a list of books. To keep your code clean and organized, you want to separate the route handling logic into a controller.

🎯 Goal: Create an Express server with a controller pattern where route handlers are defined in a separate controller object. You will set up the data, configure a controller, connect routes to controller methods, and complete the server setup.

📋 What You'll Learn

Create an array called books with three book objects, each having id and title properties

Create a controller object called bookController with a method getAllBooks that sends the books array as JSON

Set up an Express route GET /books that uses bookController.getAllBooks as the handler

Complete the Express server setup to listen on port 3000

💡 Why This Matters

🌍 Real World

Separating route logic into controllers helps keep Express apps organized and easier to maintain, especially as they grow.

💼 Career

Understanding the controller pattern is essential for backend developers working with Express or similar web frameworks.

1

DATA SETUP: Create the books array

Create an array called books with these exact objects: { id: 1, title: 'The Hobbit' }, { id: 2, title: '1984' }, and { id: 3, title: 'Pride and Prejudice' }.

Express

// Your code here

Hint

Use const books = [ ... ] with three objects inside the array.

2

CONFIGURATION: Create the bookController object

Create a constant object called bookController with a method getAllBooks that takes req and res parameters and sends the books array as JSON using res.json(books).

Express

const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' },
  { id: 3, title: 'Pride and Prejudice' }
];

// Your code here

Hint

Define bookController as an object with a method getAllBooks that calls res.json(books).

3

CORE LOGIC: Set up the Express route using the controller

Import Express, create an app with express(), and add a route GET /books that uses bookController.getAllBooks as the handler.

Express

const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' },
  { id: 3, title: 'Pride and Prejudice' }
];

const bookController = {
  getAllBooks(req, res) {
    res.json(books);
  }
};

// Your code here

Hint

Use const app = express() and app.get('/books', bookController.getAllBooks).

4

COMPLETION: Start the Express server

Add app.listen(3000) to start the server on port 3000.

Express

const express = require('express');
const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' },
  { id: 3, title: 'Pride and Prejudice' }
];

const bookController = {
  getAllBooks(req, res) {
    res.json(books);
  }
};

const app = express();
app.get('/books', bookController.getAllBooks);

// Your code here

Hint

Use app.listen(3000) to start the server.

Practice

1. What is the main purpose of using the controller pattern in Express route handlers?

easy

A. To separate route logic from routing setup for better organization

B. To combine all route handlers into a single file

C. To directly write SQL queries inside route handlers

D. To avoid using middleware functions in routes

Solution

1. Step 1: Understand the controller pattern concept

   The controller pattern means moving the logic that handles requests out of the route definitions into separate functions or files.

2. Step 2: Identify the benefit of separation

   This separation improves code organization, making it easier to read and maintain.

3. Final Answer:

   To separate route logic from routing setup for better organization -> Option A

4. Quick Check:

   Controller pattern = separation of concerns [OK]

Hint: Controllers keep route files clean and logic separate [OK]

Common Mistakes:

• Thinking controllers combine all routes in one file
• Believing controllers write database queries directly
• Confusing controllers with middleware avoidance

2. Which of the following is the correct way to import and use a controller function named getUser in an Express route?

easy

A. router.get('/user', getUser());

B. router.get('/user', () => getUser);

C. router.get('/user', getUser => {});

D. router.get('/user', getUser);

Solution

1. Step 1: Understand how to pass controller functions

   In Express, route handlers expect a function reference, not the result of calling the function.

2. Step 2: Identify correct syntax

   Passing getUser without parentheses passes the function itself. Using parentheses calls it immediately, which is incorrect here.

3. Final Answer:

   router.get('/user', getUser); -> Option D

4. Quick Check:

   Pass function reference without () [OK]

Hint: Pass controller as function reference, no parentheses [OK]

Common Mistakes:

• Calling the controller function immediately with ()
• Using arrow functions incorrectly wrapping the controller
• Returning the function instead of calling it

3. Given this controller function and route setup, what will be the response when a GET request is made to /hello?

const sayHello = (req, res) => {
  res.send('Hello from controller!');
};

router.get('/hello', sayHello);

medium

A. Hello from controller!

B. Error: sayHello is not a function

C. No response, request hangs

D. 404 Not Found

Solution

1. Step 1: Analyze the controller function behavior

   The sayHello function sends a plain text response 'Hello from controller!' when called.

2. Step 2: Check route setup correctness

   The route uses sayHello correctly as the handler for GET requests to '/hello'.

3. Final Answer:

   Hello from controller! -> Option A

4. Quick Check:

   Controller sends response text [OK]

Hint: Controller sends response, route calls it correctly [OK]

Common Mistakes:

• Forgetting to call res.send inside controller
• Passing controller incorrectly causing runtime error
• Misreading route path causing 404

4. Identify the error in this Express controller pattern usage:

const userController = {
  getUser: (req, res) => {
    res.json({ name: 'Alice' });
  }
};

router.get('/user', userController.getUser());

medium

A. res.json should be res.send

B. Missing import statement for router

C. Calling userController.getUser() immediately instead of passing function

D. Route path should be '/users' not '/user'

Solution

1. Step 1: Check how controller function is passed

   The code calls userController.getUser() immediately, which runs the function and passes its result, not the function itself.

2. Step 2: Understand correct usage

   Express expects a function reference as the route handler, so it should be userController.getUser without parentheses.

3. Final Answer:

   Calling userController.getUser() immediately instead of passing function -> Option C

4. Quick Check:

   Pass function reference, not call it [OK]

Hint: Don't add () when passing controller to route [OK]

Common Mistakes:

• Calling controller function instead of passing it
• Confusing res.json and res.send (both valid here)
• Assuming route path naming causes error

5. You want to organize your Express app using the controller pattern. You have multiple routes for users: getUser, createUser, and deleteUser. Which approach best follows the controller pattern for clean code?

hard

A. Put all route definitions and logic inside a single routes.js file without controllers

B. Define all user-related functions in a separate userController.js file and import them in the router file

C. Write all user route handlers directly inside the main app.js file

D. Use inline anonymous functions for each route in the router file

Solution

1. Step 1: Understand controller pattern best practice

   Controllers group related route logic into separate files or modules to keep code organized and reusable.

2. Step 2: Evaluate options for clean code

   Defining all user-related functions in a dedicated userController.js file and importing them into the router file follows the pattern well.

3. Final Answer:

   Define all user-related functions in a separate userController.js file and import them in the router file -> Option B

4. Quick Check:

   Separate logic into controller files [OK]

Hint: Group related handlers in controller files, import in routes [OK]

Common Mistakes:

• Putting all logic in main app file making it messy
• Mixing routes and logic in one file without separation
• Using anonymous functions inline reduces reusability